There are a plurality of interferometries and interferometers which are used to make optical and non-contact measurements, but most of the interferometries and interferometers are not in common path structure. Therefore, there are following problems:
(1) In a non-common path structure, the optical path difference between the mutually interfered reference beam and test beam should be within the coherent length. Besides, the contrast of the interference signal will decrease when the optical path difference is increased. PA1 (2) In a non-common path structure, the environment of the two light beams should be strictly controlled. For example, the air disturbance and the outer vibration should be avoided. Besides, the pressure, temperature and humidity of the control room are also necessary to be controlled stably and effectively. PA1 (3) The analysis of the interference fringes is more complex, troublesome and less precise. PA1 (1) The measurement precision is easily decreased by outer light source or scattering light source when the refractive index is measured by light intensity, thus it is better to measure the medium to be tested in a darkroom so as to isolate the unnecessary light sources. Therefore, it is inconvenient to operate the interferometer in a bright room. Besides, the light source should have a very stable single wavelength. PA1 (2) The measurement of the unnecessary reflection light and scattering light among the boundary will be varied by the different incident angles, thus the error compensation of the light intensity will be more troublesome. The measurement precision will be decreased by the light source stability, medium absorption, scattering light and the reflection light from other boundary to be tested. Besides, the refractive index measurement should firstly obtain the thickness of the medium to be tested, and the interference signal is easily effected by the environment disturbance.
The heterodyne interferometry carries the physical values to be tested in the phase of the interference signal rather than the amplitude of the interference signal. Therefore, the change of the light intensity during the interfered process will not effect the measurement result. The physical value to be tested can be retrieved from the phase of the interference signal in real time, and this is the unique characteristic of heterodyne interferometry. However, if the heterodyne interferometry is not used in a common path structure, it still has disadvantages such as the above-mentioned (1) and (2) and the measurement precision will be decreased.
Refractive index is a very important characteristic parameter of optical materials. There are a plurality of measuring methods proposed such as critical angle method, prism couple method, Brewster angle method and total reflection method. All of these methods utilize the change of the light intensity of the reflection beam or transmission beam to derive the refractive index. However, such methods for measuring the refractive index of the materials to be tested by the light intensity have two main disadvantages:
In the angle measurement, it is conventionally to measure the rotation angle of a medium by mechanic scaling. However, an optical measurement, such as interferometer or autocollimator, is applied when some precision and non-contact requirements are necessary. Though the method has advantage such as high resolution, such a method requires a large space. It is hard to apply such a measurement system in a limit space.